Official: Dreamliner suffers 'minor' setback

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Boeing says it is aware of the issue, which was caused during maintenance

Both companies say it is not related to the battery issue that grounded Dreamliners

A Japanese operator of Boeing's 787 Dreamliners said it suffered a "minor" setback while conducting a test flight of an aircraft this month.

All Nippon Airways, the Japanese airline that is the largest operator of Boeing's beleaguered Dreamliner, said an electric distribution panel overheated and blackened during a 787 test flight recently.

The May 4 test flight landed safely, and the cause of the "minor" incident was that a nut was not properly tightened during maintenance by Boeing workers, said Megumi Tezuka of All Nippon Airways.

A Boeing spokeswoman said: "We are aware of the event, which was resolved quickly as noted by ANA."